1. Field of the Invention
This invention relates generally to bifocal and other multi-focal intraocular lenses, their production, and to their implantation and use in the eye. In particularly preferred embodiments, the invention relates to the use of intraocular lenses in aphakia, pseudophakia, anterior cortical cataract extraction (acce), posterior cortical cataract extraction (pcce), accommodative restorative surgery for presbyopes, and in refractive correction surgery.
2. Description of Related Art
A general discussion of the human eye physiology will be provided for the purpose of furthering an understanding of this invention. Generally, the most outwardly visible structures of the human eye include an optically clear anterior cornea, the iris sphincter sitting behind the cornea, and the aperture of the iris, which aperture is referred to as the pupil. The pupil usually appears as a circular opening concentrically inward of the iris. Light passes through the pupil along a path to the retina in the back of the eye. In a healthy human eye, a physiological crystalline lens with a capsular bag is positioned posterior to the iris. The chamber between the posterior cornea and the front surface of the capsular bag is commonly referred to in the art as the anterior chamber. A posterior chamber is the area behind the anterior chamber, and includes the capsular bag and physiological crystalline lens.
Ciliary muscle concentrically surrounds the capsular bag, and is coupled to the physiological crystalline lens by suspensory ligaments, also known as zonules. Vitreous humor is contained in the posterior chamber behind the capsular bag. The vitreous humor is surrounded by the retina, which is surrounded by the sclera. The functional and interrelationship of these structures of the human eye are well known in the art and, for this reason, are not elaborated upon in detail herein, except as is needed or useful for facilitating an understanding of this invention.
Light entering the emmetropic human eye is converged towards a point focus on the retina at a point known as the fovea. The cornea and tear film are responsible for the initial convergence of entering light. Subsequent to refraction by the cornea, the light passes through the physiological crystalline lens, where the light is refracted again. When focusing on an object, ideally the physiological crystalline lens refracts incoming light towards a point image on the fovea of the retina. The amount of bending to which the light is subjected is termed the refractive power. The refractive power needed to focus upon an object depends upon how far away the object is from the principle planes of the eye. More refractive power is required for converging light rays to view close objects clearly than is required for converging light rays to view distant objects clearly.
A young and healthy physiological lens of the human eye has sufficient elasticity to provide the eye with natural accommodation ability. A young elastic lens may alter its shape, by a process known as accommodation, to change refractive power. The term accommodation refers to the ability of the eye to adjust focus between the distant point of focus, called the Punctum Remotum or pr (far point beyond 20 feet or 6 meters away), and the near point of focus called the Punctum Proximum or pp (near point within 20 feet or 6 meters away from the eye). Focus adjustment is performed in a young elastic lens using the accommodative-convergence mechanism. The ciliary muscle functions to shape the curvature of the physiological crystalline lens to an appropriate optical configuration for focusing light rays entering the eye and converging the light on the fovea of the retina. It is widely believed that this accommodation is accomplished via contracting and relaxing the ciliary muscle, which accommodate the lens of the eye for near and distant vision, respectively.
More specifically, the eye is “unaccommodated” for far vision by the ciliary muscle relaxing to decrease the convexity of the lens, according to accepted theoretical models of the function of the accommodative mechanism. In this unaccommodated state, the ciliary muscle relaxes, the suspensory zonules holding the lens in place and anchoring it to the ciliary muscle are at their greatest tension. The tension of the zonules causes the lens surfaces to take their flattest curves, making the retina conjugate with the far point pr. On the other hand, the ciliary muscle actively accommodates the eye for near vision by increasing the convexity of the lens within the eye via contraction of the muscle. In the accommodated state, the ciliary muscle is constricted in a sphincter-like mode, relaxing the zonules and allowing the lens to take a more convex form. In the fully accommodated state, the retina is coincident with the near point of accommodation pp. The maximum accommodative effort is termed the amplitude.
The term emmetropia is understood in the art to mean that natural focus of the optics of the eye when viewing a distant object (greater than 6 meters) is coincident with the retina. The term ammetropia means that the distance focus is displaced from the retina, such as in the case of hypermetropia, astigmatism, and myopia. Hypermetropia denotes an error of refraction caused when the retina intercepts the rays (or pencils) received by the eye before the rays reach their focus. Myopia denotes an error of refraction caused when the pencils within the eye focus to a real point before the pencils reach the retina.
According to one theory, the physiological crystalline lens slowly loses its elasticity as it ages. As the physiological crystalline lens ages, the alteration in curvature becomes less for the same action of the ciliary muscle. According to another theory, the physiological lens enlarges with age causing a decrease in working distance between the ciliary body and the lens, resulting in decreased focus ability for the same muscle action. For most people, generally the decline in focusing ability starts in youth and continues until the age of about 60. Generally, it becomes necessary for most people around the age of 40 to use near addition lenses to artificially regain sufficient amplitude at near to accommodate for the pp when attempting to perform near-point activities such as reading. This condition is known as presbyopia, and afflicts almost every human being.
With presbyopia, incoming light rays from the pp are focused at a virtual point situated behind the retina. The ciliary body-zonules-lens complex becomes less efficient at accommodating the focus of these rays on the retina. Convergence of the rays in a healthy, phakic (with lens) eye having presbyopia is most commonly achieved with the assistance of eyeglass lenses, contact lenses, or refractive surgery. Distance and near objects can then be seen clearly.
Aphakia is the condition in which the crystalline lens is either absent or, in very rare cases, displaced from the pupillary area so that it adversely affects the eye's optical focusing system. The former condition may be congenital, but it is usually the result of cataract-removal surgery. With advancing age, the physiological crystalline lens tends to develop opacities—a condition known as cataractogenesis—which unless treated eventually leads to blindness.
In the absence of other pathology or degenerative changes, removal of the opaque crystalline lens afflicted with cataracts restores the possibility of obtaining good vision with refractive implements such as eyeglasses, contact lenses, or intraocular lenses. Pseudophakia occurs when the crystalline lens is replaced with a synthetic intraocular lens.
Removal of the crystalline lens by surgery entails the loss of ability to accommodate, so additional positive power in the form of a near addition is needed for near focus. If the synthetic lens is of proper power and results in the pr focusing on the retina, the refractive error for distance will have been eliminated. However, current synthetic intraocular lenses lack the flexibility of a physiological crystalline lens. As a consequence, it is difficult, if not impossible, for the ciliary muscle to focus current synthetic intraocular lenses in the same way as a physiological lens to adjust for objects near the pp. Thus, conventional monofocal intraocular lenses provide little, if any accommodating ability.
Generally, a plus-powered eyeglass lens or contact lens is used in conjunction with an eye having a synthetic intraocular lens to adjust for objects near the pp. Pseudophakic individuals corrected for distance and emmetropia will usually require a lens in front of their eye the equivalent of approximately +2.50 diopters of power to be able to focus on near-point objects between 12 and 20 inches from the eye (approximate). However, “reading” glasses and contact lenses have the drawbacks of being inconvenient, uncomfortable, susceptible to loss and breakage, and in the case of glasses, aesthetically undesirable to some users.
Several synthetic intraocular lenses exist with zones that alter near focus powers with distance, claiming to assist the pseudophake with viewing near objects. An example of such an intraocular lens is U.S. Pat. No. 5,344,448. One problem with these designs is the zones of far and near are present simultaneously on the retina, thereby resulting in some blur or visual distortion at distance and near.
An intraocular lens that uses multiple fluids of different refractive indices is disclosed in U.S. Pat. No. 4,720,286. The intraocular lens of the '286 patent is comprised of a solid transmissive material having a hollow lenticule that encompasses the optical zone of the eye. By moving fluids of different indices of refraction through the lenticule, the lens can be made to change its power. A major drawback of the '286 patent and like structures is that channels and reservoirs are needed to translate one of the fluids away from the optical axis while translating the other fluid to the optical axis. For example, intraocular lens of the '286 patent has fluid reservoirs above and below the lenticle, and channels on both sides of the lenticle for interconnecting the reservoirs. The existence of interior or exterior channels and reservoirs increases lens production expenses, makes the intraocular lens more susceptible to damage, and may impede or prevent the folding of the intraocular lens. Lens folding and deformation is often desirable during implantation of the lens into the eye. The thinness of the channels also may increase surface tension to prevent the fluids from creating the desired accommodative effect.
The inventor is unaware of any existing intraocular lens capable of effectively and actively altering focus from distance to near and back in presbyopic or pseudophakic individuals by utilizing the natural movement of the human eye and/or head. Attempts to create a “focusing” intraocular synthetic lens have been less than successful, and presbyopia, whether age-related or in pseudophakia, continues to be a vexing problem within eye care with no highly successful solutions yet in existence.
Another drawback to intraocular lenses is that an eye that has received an implant for restoring the natural accomodation of the eye may have its refractive error changed by the process of the implantation itself. In this event, the focusable implant may function properly, but the powers needed to achieve clear distance focus may not be the same as calculated prior to insertion of the implant. For example, an IOL selected for an eye of a particular length that is to be operated on might not function to fully correct maximum distance vision after the eye has healed. As the eye heals, the lens may settle off-axis, tilt, or translate further forward or backward than the surgeon intended, leaving a refractive error that will make it necessary for the patient to use distance corrective lenses to see clearly.